1. Field of the Invention
The present invention relates to a marine propulsion device, and more particularly, it relates to a marine propulsion device including a cooling water pump configured to pump cooling water to cool an engine.
2. Description of the Related Art
A marine propulsion device including a cooling water pump configured to pump cooling water to cool an engine is known in general. Such a marine propulsion device is disclosed in Japanese Patent Laying-Open No. 2013-107538, for example.
Japanese Patent Laying-Open No. 2013-107538 discloses an outboard motor (marine propulsion device) including an engine, a drive shaft coupled to the engine and configured to transmit power, and a cooling water pump configured to be driven by rotation of the drive shaft and to pump cooling water to cool the engine. In the outboard motor according to Japanese Patent Laying-Open No. 2013-107538, a main water intake and a subordinate water intake are provided, and water taken in from both of the water intakes is combined in a common cooling channel such that the degree of freedom of the layout of a cooling water path is ensured, and the efficiency of supplying water to the cooling water pump is enhanced.
In the outboard motor (marine propulsion device) according to Japanese Patent Laying-Open No. 2013-107538, the efficiency of supplying water to the cooling water pump is enhanced, but the amount of cooling water discharged from the cooling water pump is restricted by the pump capacity so that the amount of cooling water discharged from the cooling water pump cannot be increased even in the case where the capacity of the cooling channel on the upstream side with respect to the cooling water pump is increased.
Therefore, it has been proposed to increase the pump capacity of the cooling water pump to increase the discharge rate of the cooling water.
In the case where the length of the cooling water pump in a radial direction is increased to increase the pump capacity, however, the speed of an outer peripheral portion of the cooling water pump is increased so that cavitation is easily generated. Consequently, an increase in the amount of discharge (flow rate) is hindered, and the durability of the cooling water pump is reduced by erosion. Therefore, it is difficult to increase the amount of discharge of the cooling water while maintaining the durability of the cooling water pump.